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Upsilon production in pp and pA collisions: from RHIC to the LHC

J. P. Lansberg

TL;DR

Problem: elucidate Upsilon production in pp and pA across RHIC–LHC energies and quantify cold nuclear matter effects. Approach: use the Colour-Singlet Model at LO/NLO with NNLO* guidance for high-$P_T$, comparing to LHC data and exploring cross-section ratios; analyze pA effects at RHIC/LHC. Key findings: LO CS reproduces total pp yields across energies; NLO describes low-$P_T$ spectra well (LHCb up to ~5 GeV); NNLO* helps at high $P_T$; cross-section ratios align with data; in pA, absorption is small but shadowing and EMC effects matter with rapidity; pPb at 5–15% anti-shadowing and 20–25% forward suppression anticipated. Impact: guides interpretation of heavy-ion results and prioritizes pPb measurements to isolate cold nuclear matter effects.

Abstract

I discuss Upsilon production in pp collisions at RHIC, Tevatron and LHC energies, in particular the behaviour of the differential cross section in rapidity and the impact of QCD corrections on the P_T differential cross section. I also emphasise the very good agreement between the parameter-free predictions of the Colour-Singlet Model (CSM) and the first LHC data, especially in the region of low transverse momenta, which is the most relevant one for heavy-ion studies. I also show that the CSM predicts Upsilon cross-section ratios in agreement with the most recent LHC data. I then briefly discuss the nuclear-matter effects on Upsilon production at RHIC and the LHC in p(d)A collisions and, by extension, in AA collisions. I argue that a) the Upsilon break-up probability can be neglected, at RHIC and the LHC, b) gluon shadowing --although non-negligible-- is not strong enough to describe forward RHIC data, c) backward RHIC data hints at a gluon EMC effect, possibly stronger than the quark one. Outlooks for the LHC pPb run are also presented.

Upsilon production in pp and pA collisions: from RHIC to the LHC

TL;DR

Problem: elucidate Upsilon production in pp and pA across RHIC–LHC energies and quantify cold nuclear matter effects. Approach: use the Colour-Singlet Model at LO/NLO with NNLO* guidance for high-, comparing to LHC data and exploring cross-section ratios; analyze pA effects at RHIC/LHC. Key findings: LO CS reproduces total pp yields across energies; NLO describes low- spectra well (LHCb up to ~5 GeV); NNLO* helps at high ; cross-section ratios align with data; in pA, absorption is small but shadowing and EMC effects matter with rapidity; pPb at 5–15% anti-shadowing and 20–25% forward suppression anticipated. Impact: guides interpretation of heavy-ion results and prioritizes pPb measurements to isolate cold nuclear matter effects.

Abstract

I discuss Upsilon production in pp collisions at RHIC, Tevatron and LHC energies, in particular the behaviour of the differential cross section in rapidity and the impact of QCD corrections on the P_T differential cross section. I also emphasise the very good agreement between the parameter-free predictions of the Colour-Singlet Model (CSM) and the first LHC data, especially in the region of low transverse momenta, which is the most relevant one for heavy-ion studies. I also show that the CSM predicts Upsilon cross-section ratios in agreement with the most recent LHC data. I then briefly discuss the nuclear-matter effects on Upsilon production at RHIC and the LHC in p(d)A collisions and, by extension, in AA collisions. I argue that a) the Upsilon break-up probability can be neglected, at RHIC and the LHC, b) gluon shadowing --although non-negligible-- is not strong enough to describe forward RHIC data, c) backward RHIC data hints at a gluon EMC effect, possibly stronger than the quark one. Outlooks for the LHC pPb run are also presented.

Paper Structure

This paper contains 7 sections, 1 equation, 2 figures.

Table of Contents

  1. Introduction
  2. $\Upsilon$ production in $pp$ collisions: from RHIC to LHC energies
  3. Total and differential cross sections
  4. Cross-section ratios at LO
  5. $\Upsilon$ production in $pA$ collisions: from RHIC to LHC energies
  6. $d$Au collisions at RHIC
  7. Outlooks for $p$Pb collisions at the LHC

Figures (2)

  • Figure 1: (a) and (b): comparison between the CSM predictions for the direct $\Upsilon(1S)$ yield and various experimental data Abelev:2010amKhachatryan:2010zgAaij:2012ve for the prompt $\Upsilon(1S)$ yield multiplied by $F^{\rm direct}_{\Upsilon(1S)}$Affolder:1999wm or $F^{\rm direct}_{\Upsilon(1S+2S+3S)}$Brodsky:2009cf for the STAR data. (c): comparison between the $\Upsilon(3S)$ LHCb data Aaij:2012ve and the NLO and NNLO$^\star$ CSM predictions for the direct yield.
  • Figure 2: $\Upsilon$ cross-section ratios "$3S/1S$" and "$2S/1S$" as measured by LHCb Aaij:2012ve and CMS Khachatryan:2010zg and as predicted by the CSM without the kinematical effect mentioned in the text. The systematic experimental uncertainties from the unknown polarisation are not shown.